Numerous examples of such annular friction liners are known, which are in the form of a flat annular ring, and which are used for equipping a clutch friction wheel or clutch disc, or a disc brake, in either case using dry friction in operation. Each friction liner, in the form of a flat annular ring, is made in the form of a workpiece, that is to say an embryonic or partly finished liner, which undergoes a final curing operation in order to obtain polymerisation of some of its constituents, and this curing operation may in appropriate cases be followed by a post-curing operation.
The flat annular workpiece may, in accordance with various known methods, consist for example of threads which are impregnated with a thermosetting resin, and which are cut to variable lengths of between 5 and 30 mm, the workpiece being preformed in a mould.
Alternatively, it may be based on threads which are impregnated and braided or plaited, the length of the braid or plait corresponding to the outer diameter of the workpiece.
The workpiece may consist of threads which are impregnated and woven, so as to obtain a band of fabric the length of which corresponds to the outer diameter of the workpiece.
Again, the workpiece may be made from impregnated threads which are wound or rolled on a mandrel to the internal diameter of the workpiece, the tube thus obtained by winding being subsequently cut off, or sliced, into lengths, so as to produce a corresponding number of flat annular workpieces to be subsequently completed by curing.
Finally, the workpiece may be made from impregnated threads which are wound or rolled by superimposition of lobes corresponding to the outer diameter of the workpiece.
All the above methods of making the unfinished liners (or workpieces), as described above, have as their main drawback the fact that they result in the production of annular friction liners which have low centrifugal strength, especially under hot conditions, that is to say they have poor resistance to rupture under the effects of centrifugal force; this effect is more marked the higher the working temperature.
Rupture under centrifugal stress is in general due to the fact that the maximum tangential stress in the region of the cylindrical inner surface of the crown-shaped liner exceeds the tensile rupture stress of the friction material of which it is made.